Recognition of Need
A class of three-dimensional imaging and measurement applications now requires unprecedented demonstration of capability to support new microelectronic and micromechanical fabrication technologies. For example, emerging semiconductor fabrication technologies are directed toward establishing a high density of interconnection between the chip and package. The "bumped wafer" and miniature ball grid array (".mu.-BGA") markets are emerging, and large scale growth is predicted. For instance, NEMI (National Electronics Manufacturing Initiative) has clearly indicated that the miniature array technologies are to replace traditional wire bonding interconnects. Manufacturers are experimenting with new processes. Measurement tools to support their efforts will require versatility.
For example, "dummy wafers" are used for many experiments, which have a specular and featureless surface onto which interconnects are placed. The appearance is much different than patterned wafers seen in typical production environments. This imaging phenomena is of little concern to the process engineer. In fact, the most difficult imaging problems may coincide with the best choice of process. Industry process development engineers indicate that reflowed spherical solder bumps with a smooth surface finish, sometimes a nearly perfect mirror, may be the preferred technology for the chip interconnects. The surface reflectance will vary because of process engineers' choices of relative content of lead and tin. Such targets are often "uncooperative."
The chips onto which the balls are placed are subsequently attached to printed circuit boards where both flattened and spherical mating interconnects can be expected, with either a dull or smooth surface finish. All combinations are expected. Other geometric shapes (wire with flat top, cones) can be expected in the future which will pose measurement challenges, particularly when the surface is specular with spherical or cylindrical geometry, including concavities.
Such "non-cooperative" targets, (i.e. those which present challenges for measurement systems as a result of light reflection, scattering, and geometry), are and will continue to be growing in occurrence for semiconductor, micromachining, and mass storage imaging applications. A specific growing need is recognized for an imaging system capable of improving dimensional measurement of .mu.-BGAs and bumped wafers (i.e. "spherical mirrors" on variable wafer backgrounds) and other such targets, which are "non-cooperative" with respect to traditional imaging systems. As inspection and measurement requirements for industries requiring microscopic measurement capabilities, for instance semiconductor and mass storage, become more demanding, extraordinary versatility will be needed for handling wide variation in scale, target geometry, and reflectivity. Similarly, inspection and measurement of circuit boards and the dielectric and conductive materials requires a versatile imaging system, particularly for fine geometries and densely populated component boards.
As mentioned previously, imaging requirements for the semiconductor packaging industry include defect detection as part of Package Visual Inspection (PVI), measurement of .mu.-BGA height, coplanarity, diameter, and wafer defects. High resolution and image clarity obtained from reduction of image artifacts are both required for adequate process characterization. Problems similar to those in the semiconductor area are also present when measuring other miniature parts like micromachined (micromechanical) assemblies, like miniature gears and machines, and components utilized in the mass storage industry, including substrates, disk heads, and flexures.
For example, as illustrated in FIGS. 1 and 7, inspection of a very fine solder bump or ball 20 with a "pin" or tip 22 necking down to about 1-3 .mu.m in dimension mounted on a solder pad 24, poses a measurement problem. Manufacturers often examine the tip 22 with an electron microscope for initial evaluation, but such a tool is much too slow for detailed process characterization or real time control.
Also, detection of small "hairline" burrs on IC leads is often successful using gray or 3D data using only triangulation, but false alarms are common because background noise and reflection from a container, such as a tray wall 26, can appear similar to the defect such as a burr 27, as illustrated in FIG. 2. Conversely, IC leads 28 of an IC chip 30 may be indistinguishable from the background noise 32. These false alarms are unacceptable and lower yields, thereby decreasing the value of inspection equipment.
.mu.-BGA inspection can be roughly equivalent to measuring a tiny "spherical mirror" (solder ball) mounted on a plane "mirror" (wafer) background; yet, in other cases, where the wafer is patterned and the ball has a lower tin content, is a completely different imaging problem. Solutions to such measurement problems will require versatility for handling the geometric shape and reflectance variation.
Hence, with wafer scale and other sub-micron measurement tasks, the challenges with material properties will grow, not diminish. There is a need to measure substrates, conductors, and thickness of films, or the geometry of micromechanical assemblies such as miniature gears having deep, narrow dimensions and varying optical properties, including partially transparent layers.
Prior Art Technology
Early work on defect detection of features having specular components using camera-based inspection is described in U.S. Pat. No. 5,058,178 and the references cited therein. The method is primarily directed toward lighting and image processing methods for defect detection of bumped wafers. The lighting system included combinations of bright and dark field illumination. Measurement of the diameter can be done with a camera system and appropriate illumination, but accuracy is often limited by light scattering and limited depth of focus when high magnification is required. However, in addition to defect detection and bump presence, there is a need to measure the three dimensional geometry of the bumps for process characterization. The bumps must be coplanar to provide a proper connection, and the diameter within tolerance for a good connection with the bonding pads.
Triangulation is the most commonly used 3D imaging method and offers a good figure of merit for resolution and speed. U.S. Pat. Nos. 5,024,529 and 5,546,189 describe the use of triangulation-based systems for inspection of many industrial parts, including shiny surfaces like pins of a grid array. U.S. Pat. No. 5,617,209 shows an efficient scanning method for grid arrays which has additional benefits for improving accuracy. The method of using an angled beam of radiant energy can be used for triangulation, confocal or general line scan systems. Unfortunately, triangulation systems are not immune to fundamental limitations like occlusion and sensitivity to background reflection. Furthermore, at high magnification, the depth of focus can limit performance of systems, particularly edge location accuracy, when the object has substantial relief and a wide dynamic range (i.e. variation in surface reflectance). In some cases, camera-based systems have been combined with triangulation systems to enhance measurement capability as disclosed in the publication entitled "Automatic Inspection of Component Boards Using 3D and Grey Scale Vision" by D. Svetkoff et al., PROCEEDINGS INTERNATIONAL SYMPOSIUM ON MICROELECTRONICS, 1986.
Confocal imaging, as originally disclosed by Minsky in U.S. Pat. No. 3,013,467, and publications: (1) "Dynamic Focusing in the Confocal Scanning Microscope" by T. Wilson et al.; (2) "Digital Image Processing of Confocal Images" by I. J. Cox and C. J. R. Sheppard; (3) "Three-Dimensional Surface Measurement Using the Confocal Sensing Microscope" by D. K. Hamilton and T. Wilson; (4) "Scanning Optical Microscope Incorporating a Digital Framestore and Microcomputer" by I. J. Cox and C. J. R. Sheppard; and (5) "Depth of Field in the Scanning Microscope" by C. J. R. Sheppard and T. Wilson, is similar to computerized tomography where slices in depth are sequentially acquired and the data is used to "reconstruct" a light scattering volume. In principle, an image is always formed of an object at a focal plane as taught in elementary physics, but over a region of depth there are an infinite number of planes which are out of focus yet return energy. That is to say that the lens equation for image formation is based on an idealization of an "object plane" and "image plane".
In the case of conventional confocal imaging, the slices are determined from the in-focus plane, and out-of-focus light (in front and back of the focal plane) is strongly attenuated with a pinhole or slit. Typical confocal systems use fine increments for axial positioning for best discrimination between adjacent layers in depth, for example, semi-transparent biological samples. However, the method need not be restricted to the traditional transparent or translucent objects, but can be applied both as a depth measurement tool and image enhancement method using reflected light for contrast improvement through stray light rejection. As with any method, there are advantages and disadvantages.
Application of confocal imaging to semiconductor measurement is disclosed in U.S. Pat. Nos. 4,689,491, 5,479,252 and 5,248,876. Operation of several confocal systems is described in U.S. Pat. Nos. 4,827,125; 4,863,226; 4,893,008; 5,153,428; 5,381,236; 5,510,894; 5,594,235; and 5,483,055 and H 1,530. Much of the recent work is directed toward improvements, resulting in reduction of the image memory storage requirements (store maximum, not volume), improving the efficiency and fine positioning capability of autofocus systems (coarse/fine search), exposure control for improved dynamic range, and some image enhancement methods.
Similarly, variations in confocal acquisition methods are taught in the art to solve specific problems or optimize designs for specific applications as taught in U.S. Pat. Nos. 5,239,178 and 4,873,653. However, present confocal systems are constrained by sequential slicing of the volume, whereas triangulation systems detect the top surface of the volume (profile) directly resulting in much higher speed.
In U.S. Pat. No. 5,448,359 such speed limitations are partially circumvented by utilizing a plurality of detectors and spatial filters in the confocal receiver optical path. A circuit to locate the detector producing maximum intensity is disclosed.
Similarly, USSR patent document No. 868,341 discloses a plurality of detectors with apertures (confocal) and electronic circuitry to obtain focus (3D) information about objects. The intensity of each detector is compared and used to adjust the position of the imaging system along the optical axis so as to clear the mismatch. In each case, a tradeoff is determined between depth sensitivity, complexity, and measurement speed.
Other approaches to imaging of "non-cooperative" targets, many directed toward solder joint inspection, have been proposed to measure depth or fillet shape. These are described in the Chen et al. U.S. Pat. No. 5,118,192 and a Nagoya solder joint inspection system described in "NLB Laser Inspector--NLB-7700M Specifications" by Nagoya Electric Works Co., Ltd. 1994. The system uses specularly reflected light to examine the shape of solder fillets, and to determine presence/absence of solder. Figure E in Section 6 thereof shows a missing fillet and the signals received from a plurality of detectors. A detector 6 corresponds to an "on-axis" detector, and the information is useful for estimating the diameter of the solder bump. For instance, the detector 6 receives a large signal near the top of the ball, a weak signal from the curved edge, and typically a strong signal from the area adjacent to the bump. However, narrow angle multiple reflections from the edge of the ball can corrupt the measurement and result in ambiguous edge locations. Furthermore, the sensitivity of the system may not be adequate to determine the height of regions which do not have a substantial specular reflection component.
Similarly, a recent version of the IPK solder joint inspection system manufactured by Panasert includes a coaxial detector with a triangulation-based sensing system as illustrated in their brochure entitled "IPK-V" believed to be published in 1997. The .mu.-BGA, bumped die, and numerous other problems range from scenarios where prior art technology is adequate, but in many cases unacceptable, and even inoperable conditions exist.
Wafer measurement and defect detection systems have utilized multiple detectors advantageously. U.S. Pat. No. 5,416,594 describes a system which uses both reflected and scattered light for detection of defects and thin film measurements. The reflected beam is received at an angle of reflection which is non-collinear with the transmitted beam and the scattered light is collected over a relatively large angle which excludes the reflected beam energy. The scattered light beam, representative of surface defects, may be collected at an angle which is widely separated (more than 30 deg.) from the incident beam. The off-axis illumination and the corresponding reflected beam are utilized for film thickness measurements, sometimes with multiple laser wavelengths. The scattered light signal is analyzed in conjunction with that representing the reflected light. Although the imaging geometry is well matched to the specific cited inspection requirements, there are several potential disadvantages encountered when attempting to simultaneously provide information about surface defects and say, the peak height of interconnects like solder bumps (which have substantial height) and the corresponding diameter and shape.
Commercial success has not been widespread, although many approaches have been proposed. Hence, there is a need for a system and method for three-dimensional imaging capable of performing with both "cooperative" and "non-cooperative" targets. To be useful, the method and system must be accurate, robust, and have high measurement speed, the latter being a traditional limit to the use of widespread confocal imaging.